The CDC wants your semen (if you've had Zika)

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Photos:Zika worries: Who's skipping the Olympics?

Milos Raonic is the first leading tennis player to pull out of the Rio 2016 Olympics because of "uncertainty" over the Zika virus. The world No. 7 withdrew from Canada's team on July 15.

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Photos:Zika worries: Who's skipping the Olympics?

Young golf star Jordan Spieth has pulled out of contention to take part at the 2016 Rio Olympics. The American cited fears about the Zika virus, according to the International Golf Federation, which released a list of eligible players on July 11.

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Photos:Zika worries: Who's skipping the Olympics?

The U.S. golf team will also be without world No. 2 Dustin Johnson, who won the U.S. Open in June. Johnson pulled out on July 8, saying "my concerns about the Zika virus cannot be ignored." He already has a baby boy with fiancee Paulina Gretzky.

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Photos:Zika worries: Who's skipping the Olympics?

South African golfer Lee-Anne Pace, who ranks No. 21 in the LPGA, said she does not want to be considered to represent her country in Rio this summer because of Zika. Noting that the decision is personal, she said, "Playing in the Rio 2016 Olympics is an incredible honor for any athlete, and we are excited for golf's return to the Games. We also realize that the Zika virus is a concern for many, particularly for women with plans for a family in the near-term."

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Photos:Zika worries: Who's skipping the Olympics?

Australian Jason Day, ranked No. 1 in the world of golf, says he will not compete in the 2016 Summer Olympic Games in Rio due to concerns over the Zika virus. "The reason for my decision is my concern about the possible Zika virus and the potential risks that it may present to my wife's future pregnancies and to the future members of our family," Day said in announcing his decision.

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Photos:Zika worries: Who's skipping the Olympics?

Golf's world No. 4, Rory McIIroy, announced in June that he wouldn't be competing in the Rio Olympics amid fears surrounding the Zika virus. "After speaking with those closest to me, I've come to realize that my health and my family's health comes before anything else," McIIroy said in a statement.

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Photos:Zika worries: Who's skipping the Olympics?

On June 28, Irish golfer Shane Lowry announced he is withdrawing from the 2016 Olympics games being held in Rio de Janeiro, Brazil, in August. In a statement Lowry said, "While I am bitterly disappointed to be missing out on that experience and the opportunity to win an Olympic medal for Ireland, on this occasion I have to put my family's welfare first."

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Photos:Zika worries: Who's skipping the Olympics?

Australian golfer Marc Leishman pulled out of the Olympic Games amid concerns over the Zika virus' impact on his wife's compromised immune system. "We have consulted with Audrey's physician and, due to her ongoing recovery and potential risks associated with the transmission of the Zika virus, it was a difficult yet easy decision not to participate," he said in June.

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Photos:Zika worries: Who's skipping the Olympics?

Fiji's Vijay Singh has chosen not to participate in the Olympics, which includes golf for the first time in 112 years. Singh, a three-time major winner, was one of the first notable athletes to drop out of the games because of the Zika virus. He announced his decision in mid-April.

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Photos:Zika worries: Who's skipping the Olympics?

American cyclist Tejay van Garderen will be skipping the Olympics. "If my wife wasn't pregnant right now, I'd be going to Rio," he told CNN. "My biggest concern is for the baby on the way. I would never tell any athlete who's worked their butt off for four years not to go to the games."

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Photos:Zika worries: Who's skipping the Olympics?

"Today" anchor Savannah Guthrie announced in June that she is expecting her second child and will not be heading to Brazil to cover the Olympic Games because of concerns about the Zika virus. "I'm not going to be able to go to Rio," she told co-anchor Matt Lauer. "The doctors say we shouldn't because of the Zika virus."

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Story highlights

The Centers for Disease Control and Prevention is collecting semen from hundreds of U.S. men who had Zika

A second study in Puerto Rico is the largest collection ever of bodily fluids from Zika patients

(CNN)The Centers for Disease Control and Prevention is collecting semen from hundreds of men in the United States in order to figure out how long the dreaded Zika virus lasts in the bodily fluid.

The virus can be transmitted sexually, and it's been known to linger in semen long after a man's fever, rash and itchy eyes have gone away.

If a man has the virus in his semen and has sex with a woman who is pregnant or becomes pregnant, the baby could be born with devastating neurological birth defects.

After about two months of recruiting, some 40 men who've had Zika have volunteered to donate their semen. The CDC hopes to bring in about 210 more.

The men are asked to make about a dozen donations in their homes every other week for six months after their illness, and each time are given a $50 multi-use gift card.

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A courier picks up the donations, which are then delivered to the CDC's labs in Fort Collins, Colorado.

"I'm happy to say patients really have been quite receptive about volunteering their specimens," said Dr. Paul Mead, the senior epidemiologist at the CDC who is running the study. "They seem to understand the importance of the study."

That advice is based on studying the semen of just three men who had Zika, which is why the CDC is doing the larger study.

Separately, the CDC is doing a study in Puerto Rico that will examine the semen of men who contracted Zika but never actually had symptoms of the disease -- an important group since many who become infected have only mild symptoms or none at all, according to the CDC.

In addition to looking at the virus in semen, the Puerto Rico study will also be examining blood, saliva and vaginal secretions.

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The CDC plans to recruit 300 men and women in Puerto Rico who've had Zika and 1,000 people who live in the same households.

"The response has been amazing. People have welcomed strangers to come and get their bodily fluids," said Dr. Gabriela Paz-Bailey, the investigation's lead researcher.

She said the Puerto Rico study is the largest collection ever of bodily fluids from Zika patients.

"Nearly everyone we've approached has agreed to participate," Paz-Bailey said. "They understand that many things are not yet known."